Chiral-Induced Spin Selectivity: A Symmetry Analysis of Electronic Transmission

An intriguing phenomenon has emerged in the past years showing considerable promise for new spintronic devices, catalysis, and for biological electron transfer: the chiral- induced spin selectivity (CISS) effect, which describes the spin-filtering ability of diamagnetic helical structures like DNA or peptides having chiral symmetry. The effect is attributed to atomic spin-orbit coupling, which is surprising due to the lack of heavy elements in such compounds. Theoretical descriptions with model Hamiltonians are able to explain the spin filtering qualitatively, but underestimate its magnitude by several orders when using realistic parameters for atomic spin-orbit coupling. To find the origin of the large spin filtering, a first-principles description is mandatory. We are taking an essential step into this direction by calculating spin polarization within the Landauer–Imry– Büttiker approach, using two-component density-functional theory including spin–orbit coupling. We identify the imaginary part of the effective single-particle Hamiltonian as the origin of direction-dependent spin filtering in helical structures by showing its effect on the symmetry of the Green’s function matrices. This imaginary part originates from spin–orbit coupling. This relation could help identify what is currently lacking in first-principles modeling of the CISS effect. We further draw an analogy with imaginary terms in barrier scattering, which may help understand the unusually effective long-range electron transfer in biological systems.